And therein lies the problem. The game is designed so people feel they're working towards something. They feel it's like climbing stairs and at the top of the stair case lies the fruits of their labour. Hence people want that climb to be over quickly.
Now destroy that staircase. Make that a path filled with enjoyable experiences all the time. That means making the leveling experience what the end-game experience is today. Make the expedition into Sastasha feel and play like we're tackling the Binding Coils. Now instead of wanting that leveling time to be over quickly, instead of feeling you have to spend that 5 hours as a price of getting what you want, you are already getting it, while at the same time progressing towards more of it.
All this focus on end-game make us forget that "end-game" really was only born out of a desire to have more of that "game". It's the "one more turn". Turn 1 to turn 500 ("the game") is the foundation, turn 500 and after (the "end-game") is only desirable because people enjoyed the foundation upon which the extra turns are built.
Nobody is saying we should simply make it take 3 times longer to level. Not at all.